1. Field of the Invention
The present invention relates to a method of storing and retrieving data to and from a magnetic disk, and in particular to a method of storing and retrieving data to and from a magnetic disk wherein data may be encoded on a data track in a large number of polarization orientations in comparison to conventional bidirectional recording schemes.
2. Description of Related Art
Current data storage systems, such as those in computer hard disk drives, employ magnetic, optical or magneto-optical heads for recording and retrieving data to and from magnetic storage disks. Magnetic heads may include a transducer element for reading and writing the data and an air-bearing slider which supports the transducer over the disk. At present, there are three widely used head technologies in disk drives: ferrite metal in gap (MIG) technology, which uses machined ferrite ceramic cores with wire coils; inductive thin film technology; and magnetoresistive (MR) technology.
The principal elements of an inductive magnetic recording head are a magnetic core including two poles separated by a non-magnetic gap, and an electrically conductive coil wrapped or deposited in turns around the core. Data is transferred between the head and a magnetic storage disk in concentric data tracks having a radial width on the order of a few microns or less. Each track is divided into a plurality of sectors onto which the desired data is magnetically encoded. The radial track density of a given storage medium is described as a number of tracks per inch (TPI). Linear density may be described as a number of bits per inch (BPI) within a given track.
To write data to a particular sector, a current is passed through the coil, thereby inducing a magnetic field in the core. The magnetic field fringes out across the gap in arcuate flux lines and, in so doing, passes into the disk to magnetize a segment of the disk. Reversing the direction of the current reverses the polarity of the next magnetized segment as it passes by the gap of the head. Thus, referring to FIGS. 1 and 2, as the magnetic disk rotates under the head, data is laid down on data tracks 20 in segments 22 of alternating magnetic polarities (indicated by oppositely facing arrows). Conventional recording systems of this kind are referred to as longitudinal recording systems, as segments on a data track are oriented either to the left (i.e., up-track) or to the right (i.e., down-track) in the plane of the disk. Alternatively, in perpendicular recording systems, it is known to provide the recording medium with an easy axis of magnetization perpendicular to the plane of the disk. Systems of this kind utilize a different transducer element and result in segments on a data track being oriented into and out of the plane of the disk.
To read data, the previously encoded disk is again passed by the head and the reversing magnetic polarities within the segments induce reversing magnetic fields in the core. These reversing magnetic fields in the core generate correspondingly reversing currents in the coil, which are sensed and decoded into binary numbers by the drive circuitry. In contrast to an inductive disk head, which is typically designed to read and write data using a single inductive element, an MR disk head uses an inductive element to write data onto the disk and a separate MR element to read data from the disk. The MR read element incorporates a magnetoresistor whose electrical resistance changes in the presence of a magnetic field. As the encoded disk is passed by the read element, the disk drive circuitry senses and decodes the changes in electrical resistance caused by the reversing magnetic polarities.
There is a constant effort in the contemporary computer field to increase the amount of information that can be stored on a magnetic disk of given size. Conventionally, this problem has been attacked by writing data in smaller segments, to thus increase linear density, and/or by decreasing the width of a data track to increase radial density.
One method which has proved successful in increasing radial density in the magnetic tape recording industry is azimuth recording. As shown in U.S. Pat. No. 4,539,615 to Arai et al., azimuth recording utilizes xe2x80x9coddxe2x80x9d and xe2x80x9cevenxe2x80x9d transducer elements for recording xe2x80x9coddxe2x80x9d and xe2x80x9cevenxe2x80x9d data tracks, respectively, on the storage medium. In particular, as shown in FIG. 2, the gap in the odd transducer element is slanted in a first direction so as to polarize segments on one data track in a first slanted direction, and the gap in the even transducer element is slanted in a second direction so as to polarize segments on an adjacent data track in a second slanted direction. During the reading process, the even transducer element is insensitive to data on the odd data tracks, and the odd transducer element is insensitive to data on the even data tracks. In this way, the odd and even data tracks may be placed close together without a danger that data written on one track will be partially picked up by the neighboring track.
While conventional systems have attempted to increase the amount of information stored on a disk by increasing linear and radial densities, the amount of information contained in any given segment has remained unchanged. All conventional recording schemes store only a single bit (i.e., either a xe2x80x9c0xe2x80x9d or a xe2x80x9c1xe2x80x9d) in a particular segment, which state is indicated by the direction of polarization of that segment. Therefore, the amount of information which is stored into and read from a particular segment is limited to only one of two possible states.
Moreover, recording data onto segments by polarizing respective segments in completely opposite directions results in certain disadvantages at the boundary, or transition, between adjacent segments. While adjacent segments are typically illustrated, as in FIGS. 1 and 2, to have a sharp transition from one polarity to the opposite, the transition in fact takes place gradually as shown in the enlarged view of the boundary region shown in FIG. 3. When the magnetic field across the gap reverses completely from one direction to the opposite direction, there will be relatively large boundary, or transition length L1, which includes individual domains 24 oriented in both directions at the boundary reverse their direction gradually. The relatively large transition length L1 makes it more difficult to increase the linear density within a data track.
A further disadvantage to polarizing segments in opposite directions is the resultant signal-to-noise ratio (SNR) at the boundary between adjacent segments. It is known that in conventional recording schemes, the noise is concentrated at the transition between oppositely polarized segments. In particular, the media are noisiest when demagnetized, as in the center of a transition in conventional binary recording schemes (see, for example, J. C. Mallinson, A New Theory of Recording Media Noise, IEEE Trans. Magn., 27, pp 3519-3531, July, 1991). Noise power (NP) depends on the magnetic state as follows:
NP≈1xe2x88x92m2; 
where m=the magnetization (M) induced in a segment by the gap magnetic field÷the remanent magnetization (Mr) of the segment. Thus, as shown on the graphs of FIGS. 4 and 5, the noise power (NP) will be at a maximum when the media is demagnetized (i.e., M=0). This state occurs each time transducing element transitions between a negatively and positively polarized segment in conventional recording schemes.
The demagnetization which occurs with each segment transition in conventional recording has another disadvantage. According to the superparamagnetic effect, magnetic media formed of small grains tend to destabilize and lose their remanent magnetism in the presence of a demagnetizing field.
It is therefore an advantage of the present invention to store more information in polarized segments on a magnetic storage disk in comparison to conventional recording systems.
It is another advantage of the present invention to magnetize a segment on a magnetic storage disk in a plurality of different polarization orientations.
It is another advantage of the present invention to reduce the length of the transition region between adjacent segments.
It is a still further advantage of the present invention to reduce the signal-to-noise ratio at the transition between adjacent segments.
It is another advantage of the present invention to increase the thermal stability of each segment by reducing the demagnetizing field at the transition between adjacent segments.
These and other advantages are presented by the present invention which in preferred embodiments relates to a method of storing and retrieving data to and from a magnetic disk wherein data maybe encoded on a data track in a large number of polarization orientations in comparison to conventional bidirectional recording schemes. In one embodiment of the present invention, it is contemplated that a single segment may be polarized in eight different orientations, each of which being distinguishable from each other by a read/write head. Polarization of a segment in one of eight different orientations significantly increases the amount of information which can be stored in any given segment relative to conventional systems which are polarized in only one of two orientations. Thus, the amount of information which may be stored on a disk may be increased without having to alter the linear or radial density of the disk.
It is a further feature of the present invention that each of the possible orientations in a preferred embodiment of the present invention are directed down-track (i.e., no component of a polarization within each segment in the X direction points in opposite directions). This feature reduces the length of a boundary or transition region between two adjacent segments. Moreover, having no X-component of adjacent segments pointing in opposite directions, the signal-to-noise ratio at the transition region is reduced in comparison to conventional recorded data tracks having oppositely oriented polarized segments.